Herpes simplex virus type 2 (HSV-2) is one of the most common sexually transmitted infections in the world. In the U.S., it affects 16% of the adult U.S. population, even more among young adults and college students. But given that 75% to 90% of people who test positive for HSV-2 have never been officially diagnosed with genital herpes, most infected people don’t even know they carry the virus.

A significant number of HSV-2 positive people have never had symptoms like blisters and sores, which means that many people may be transmitting the virus to others unknowingly, and because they’re asymptomatic they’re unlikely to get tested. Even if they do get tested, a positive blood test doesn’t help patients understand their risk of transmission. Worse, many such patients believe that HSV-2 can’t be passed on unless they have an active herpes outbreak — a dangerous misconception.

The question is, how dangerous? How often do people with asymptomatic infections transmit the virus? According to a new study in the Journal of the American Medical Association, it may be more common than you’d think. “Most HSV-2 infections are acquired from persons without a clinical history of genital herpes,” wrote the study’s authors.

To quantify exactly how easily the virus is transmitted to someone without genital herpes, researchers from the University of Washington, Seattle, and the Fred Hutchinson Cancer Research Center instructed 498 HSV-2-infected participants, average age 41, to take genital swabs every day for at least one month. The scientists then analyzed the samples for evidence of viral shedding — conditions under which the virus was detectable in the genital area and able to spread to another person.

Of the participants, 410 had experienced an outbreak of genital herpes symptoms; 88 participants had been completely asymptomatic until that point. On average, the researchers found, those who had had past herpes outbreaks were infectious 20.1% of the time, while the asymptomatic group was found to be able to transmit the virus 10.2% of time — but nearly all of that (84%) was when these patients were symptom-free (some of these patients developed symptoms during the course of the study). Further, there was no difference in the amount of virus shed by symptomatic and asymptomatic patients.

During the study, which lasted for an average 57 days, researchers were able to detect shedding virus at least once in 68.2% of the people who had asymptomatic infections, compared with 83.4% of the symptomatic group.

Even though the asymptomatic participants were less likely to pass on HSV-2, they were still significantly infectious. “The risk of HSV-2 transmission is high from persons with unrecognized HSV-2 infection,” the authors wrote.

The researchers say the findings should be a wake-up call to public health officials in the United States, where genital herpes rates have remained stagnant over the past decade. They suggest that wider HSV-2 testing could help more people become aware of their status, and help curb transmission: “Condom use, daily valacyclovir therapy, and disclosure of HSV-2 [positive] status each approximately halve the risk of HSV-2 transmission.”